Tooth enamel contains hydroxyapatite as a major component, and usually in the mouth, the elution of phosphate ions or calcium ions from the tooth (i.e. demineralization) and the crystallization of calcium phosphate or hydroxyapatite (i.e. remineralization) are in an equilibrium state. Fluoride ions are able to prevent the tooth from decaying by inhibiting demineralization and by accelerating supply and crystallization of calcium ions and phosphate ions, i.e. remineralization of the tooth. Therefore supply of fluoride ions and calcium ions to the oral cavity makes it possible to accelerate remineralization of the tooth.
However, in the case when fluoride ions and calcium ions are incorporated in the same composition, calcium fluoride precipitates in the composition. Such preformed calcium fluoride is a powdery substance (mean particle size: several micrometers), and when this is supplied to the oral cavity, it will be scarcely adsorbed on the tooth because its particle size is too large, thereby hardly exhibiting an effect of accelerating remineralization of the tooth.
Based on this perspective, there has been proposed an oral preparation in which a calcium ion source and a fluoride ion source are made into separate compositions respectively, then the two compositions are admixed with each other in the mouth or immediately prior to introduction into the mouth, thereby forming calcium fluoride in the oral cavity. For example, there is an oral hygiene product containing a calcium ion source, a fluoride ion source and a calcium sequestering agent (Japanese Patent Laid-Open Nos. Sho 58-219107 and Hei 10-511956). However, since this oral hygiene product contains a calcium sequestering agent, the oral hygiene product has a problem that fluorine adsorption on the tooth is inhibited on the contrary by the calcium sequestering agent.
Moreover, the composition containing calcium fluoride preformed in a colloidal form has also been proposed (Japanese Patent Laid-Open No. Hei 3-72415). However, it has a problem that the stability of the colloid decreases when preserving over a long period, and it thus fails to have an effect enough to deposit calcium fluoride particles on the tooth surface.
Japanese Patent Laid-Open No. Sho 63-101312 discloses that rapid precipitation of calcium fluoride can be induced. In this case, however, it is not possible to control the rate of aggregation of the calcium fluoride particles (the primary particles) after precipitation, so that homo-aggregation after forming the primary particles rapidly progresses to form secondary particles. The secondary particles of calcium fluoride thus formed have a problem that the particle size grows too large and decreases the amount being adsorbed on the tooth.
Here, it is to be noted that the primary particle is the crystal particle of calcium fluoride made from fluoride ions and calcium ions, and the secondary particle is the particle formed by aggregation such as homo-aggregation of the primary particle.
Further, Japanese Patent Laid-Open No. Hei 10-511956 discloses that calcium fluoride formation is controlled in mouth rinses, dentifrices and gels. Namely, in order to control the calcium fluoride formation, it is proposed to contain a calcium fluoride inhibitor which causes to delay precipitation of calcium fluoride for at least about 5 seconds after mixing calcium ions and fluoride ions.
As the result of containing the calcium fluoride inhibitor, a delay of calcium fluoride aggregation (namely, the formation of secondary particle) has been achieved. However, the presence of the inhibitor also inhibits the formation reaction of calcium fluoride (namely, the formation of the primary particle), which causes a problem that the amount of calcium fluoride formation as the primary particle decreases.
Accordingly, in order to attain more efficient acceleration of remineralization, it is desired that the rate of calcium fluoride aggregation (the formation of the secondary particle) can be controlled without affecting calcium fluoride formation (the formation of the primary particle).